The overall goal of this application is to define alterations in myocardial transmembrane distribution of sodium and alterations in high-energy phosphate metabolism preceeding and during disturbances in cardiac rhythm which may be causal in sudden cardiac death. That alterations in intracellular ion concentrations are an important determinant of arrhythmias is evident from instances of ventricular fibrillation which accompany digitalis intoxication. The relationship between energy metabolism and alterations of intracellular ion concentrations in cardiac tissue may be expected to be important as ATP is the energy source for maintenance of transmembrane ion gradients. We will approach this problem through study of isolated perfused working guinea pig hearts by phosphorus-31 and sodium-23 nuclear magnetic resonance (P-31 and Na-23 NMR). In some instances P-31 and Na-23 observations will be made simultaneously in the same heart. In addition to global NMR observation of the entire heart, localized measurements will be made using surface coils. Using this technique, areas of the heart as small as 3 mm in diameter can be studied by NMR techniques. The experimental approach outlined in this application is unique in that it permits simultaneous observation of both high-energy phosphates and transmembrane sodium distribution at the same time as mechanical and electrical potential measurements are made in the same heart.